1. Field of the Invention
The present invention relates to a sputtering target for optical media such as optical discs, a method of manufacturing the same, an optical medium, and a method of manufacturing the same.
2. Related Background Art
Optical media such as optical discs (e.g., BD (blu-ray disc)-ROM, BD-R, and BD-RE) and optical cards have been known, which are manufactured by forming a reflective layer on a substrate by sputtering with a sputtering target and further forming an optical recording layer, a transparent cover layer through which light is transmitted at the time of reading and writing, and the like on the reflective layer. When the surface smoothness of the reflective layer on the sputtered film-deposition end surface side is not high in such an optical medium, favorable reflections may not be obtained upon irradiation with laser beams, so that structures of the optical recording layer, transparent cover layer, and the like may be affected adversely, whereby characteristics such as noise characteristics, for example, may deteriorate. The deterioration in characteristics such as noise characteristics is remarkable in particular in optical media having a recording layer which is constituted by first and second inorganic reaction layers and carries out recording by mixing the first and second inorganic reaction layers upon irradiation with a laser beam.
As a sputtering target suitable for manufacturing reflective layers for optical media, Japanese Patent Application Laid-Open Nos. 4-099171 and 4-026757 disclose Al and Al alloys mainly composed of Al. Japanese Patent Application Laid-Open No. 2002-015464 discloses a sputtering target and optical media using silver with high thermal conductivity or a silver alloy mainly composed of silver. Japanese Patent Application Laid-Open No. 2004-171631 discloses an optical medium equipped with a reflective film, mainly composed of Al, having high smoothness.